The catalyzed conversion of oxygenates to olefins, especially of methanol to propylene, is an attractive way of creating value because of the upgrading of the starting materials. Zeolite-based catalysts for conversion of oxygenates to olefins are described, for example, in EP 0 448 000 A1 and EP 1 424 128 A1.
A general problem in the case of use of zeolite-based catalysts in the conversion of oxygenates to olefins is that the catalysts tend to lose catalytic activity over the course of the process. This is brought about firstly by the increasing carbonization of the surfaces and pores. This arises because the by-products that form during the conversion of oxygenates to olefins can condense to form longer-chain or cyclic species and be deposited on the catalyst, resulting in masking of the catalytically active sites. Therefore, after a particular service life, what is called a regeneration is necessary, in which the carbonaceous deposits are removed from the catalyst under mild conditions. Secondly, the reaction conditions also bring about progressive dealumination of the zeolitic material. This is caused by the steam which arises, for example, in the case of use of water-containing feeds and/or in the conversion reaction of the oxygenates to olefins. The result of the dealumination is that the number of catalytically active sites gradually decreases, the catalyst is irreversibly deactivated and the conversion rate of the oxygenate used decreases.
To influence the activity, stability or selectivity, the prior art describes the modification of zeolite-based catalysts with phosphorus at different times in the production process, and the use of washing or steam treatments.
WO 2012/123558 and WO 2012/123556 describe the preparation of a phosphorus-modified zeolite-based catalyst by applying a phosphorus compound to an extruded and calcined zeolite. WO 2012/123557 describes the preparation of a phosphorus-containing zeolite-based catalyst by extrusion of a phosphorus-modified zeolite followed by a final calcination. These preparation processes do not include a steam treatment; by contrast, the catalysts obtained have to be subjected to a steam treatment prior to use in an MTO process.
U.S. Pat. No. 4,356,338 describes a process for reducing the level of carbon deposits and prolonging the service life of a zeolitic catalyst, by subjecting it to a steam treatment and/or a treatment with phosphorus compounds. This catalyst is notable for a lower tendency to carbonization in use as catalyst for the aromatization of 1-heptene, and it is simultaneously possible to observe a lowering of the output yields. The catalyst has phosphorus contents between 2% and 15% by weight.
WO 2011/044037 describes a zeolite-based catalyst which is prepared by treating a zeolite with a phosphorus compound. The phosphorus-treated zeolite is admixed with a binder, extruded, calcined and contacted with liquid water, removing a portion of the phosphorus from the phosphorus-treated zeolite. The catalyst described in WO 2011/044037, after prior steam treatment, is used in processes for alkylating aromatics.
EP 2 348 004 A1 describes a process for preparing a phosphorus-modified zeolite-based catalyst and the use of the catalyst in an MTO process. In this process, steam treatment reduces the aluminum content of a ZSM-5 zeolite. The catalyst is then prepared by applying phosphorus to the zeolite and then mixing the phosphorus-modified zeolite with one or more binders, alkaline earth metal salts, rare earth metal salts, clays and shaping additives.
WO 2009/156434 describes a process for preparing lower olefins, by providing an XTO reaction zone, an OC reaction zone and a catalyst regeneration zone, using a phosphorus-modified zeolite-based catalyst. Prior to the application of phosphorus with a solution, a zeolite is subjected to a steam treatment at a temperature of 400° C. to 870° C. for 0.01 to 200 h and optionally mixed with a binder, and a portion of the aluminum is removed by leaching with an aqueous acid solution.
WO 2007/076088 relates to a process for preparing a phosphorus-modified zeolite-based catalyst and to the use of the catalyst in a toluene methylation process. The zeolite is modified with phosphorus and then bound with an inorganic oxide binder which has been treated with mineral acid. Prior to the use in the toluene methylation process, the catalyst is treated with steam at a temperature of 300° C. or less.
The crucial disadvantage of known non-phosphorus-modified catalysts for the conversion of oxygenates such as methanol or dimethyl ether to lower olefins and specifically the conversion of methanol to propylene is still deactivation as a result of carbonization within one cycle and the dealumination of the catalysts because of the presence of water during the catalytic conversion over several cycles. It is found in the case of use of phosphorus-modified catalysts that the modification does bring about an increase in the methanol conversion rate, but the existing preparation methods, on the other hand, lead to a disadvantageous decrease in propylene yield or propylene selectivity (to a greater or lesser degree according to the modification process used and phosphorus content). However, a lowered propylene selectivity reduces the economic viability of the process.
Even with the known processes for converting methanol to propylene using non-phosphorus-modified catalysts, propylene selectivities are achieved that can still be optimized. In general, in the conversion of oxygenates such as methanol or dimethyl ether to lower olefins, especially of methanol to propylene, the propylene selectivity increases with rising temperature. On the other hand, however, deactivation as a result of carbonization and dealumination also increases drastically in processes for converting oxygenates to olefins with rising temperature. An increase in selectivity by increasing the temperature in the reactor in the conversion of oxygenates such as methanol or dimethyl ether to olefins is thus desirable, provided that the known disadvantages for the performance of the catalyst can be overcome.
It is known from the prior art that phosphorus modifications of zeolite-based catalysts can prolong the lifetime thereof. The term “lifetime” in this connection is understood to mean the duration of the catalyzed conversion to hydrocarbons until the same conversion of, for example, not less than 95% is attained. Experiments on modified catalysts prepared according to the prior art show that phosphorus modifications of formed extrudates under relevant process conditions (for example addition of water, for instance in a water:methanol weight ratio of 2:1) can have an adverse effect on olefin selectivity, particularly on the selective release of propylene. The overall cumulated propylene yield obtained over one cycle cannot be increased to a maximum degree by an increase in temperature, since either the drastic shortening of lifetime in the case of phosphorus-free catalysts or the decrease in the propylene selectivity in the case of phosphorus-modified catalysts has an adverse effect. It is thus not possible with the catalysts from the prior art to achieve a maximum increase in the olefin yield for the process for conversion of oxygenates such as methanol or dimethyl ether to olefins by increasing the temperature.